Technical Field of the Invention
The present invention relates to a liquid crystal display apparatus, a driving method for the same, and a driving circuit for the same. More particularly, the present invention relates to a high-efficiency liquid crystal display apparatus capable of responding at high speed, a driving method for the same, and a driving circuit for the same.
Description of the Related Art
With advancements in the multimedia age, a liquid crystal display apparatus from small-sized ones used in projectors, cellular telephones, view finders, and so on to large-sized ones used in notebook PCs, monitors, televisions, and so on has rapidly come into widespread use. Moreover, to electronic equipment such as viewers and PDAs and further to amusement machines such as handheld video game machines and pinball machines as well, middle-sized liquid crystal display apparatuses have become indispensable. On the other hand, liquid crystal display apparatuses have been used in all sorts of units including household appliances such as refrigerators and microwave ovens. At present, in most liquid crystal display elements, a twisted nematic (TN) display system is used. The liquid crystal display elements having the TN type display system are made of a nematic liquid crystal composition. When the conventional TN type liquid crystal display elements are subjected to simple matrix driving, it has been found that their display quality is not high and their number of scanning lines is limited. Therefore, in the simple matrix driving, an STN (Super Twisted Nematic) type liquid crystal display system is mainly used instead of the TN type liquid crystal display system. The STN type liquid crystal display system has improved contrast and viewing angle dependence when compared with initial simple matrix driving system using the TN type liquid crystal display system. However, since the STN type liquid crystal display apparatuses are low in response speed, these are not suitable for moving image displays. To improve the display performance of the simple matrix driving system, an active matrix system, in which each pixel is provided with a switching element, has been developed and widely used. For example, the TN type display apparatuses having thin film transistors (TFTs), that is, TN-TFT type display apparatuses are widely used. Since the active matrix system using the TFTs has a higher display quality than the simple matrix driving system, the TN-TFT type liquid crystal display apparatuses have become mainstream in the market at present.
On the other hand, due to a demand for even higher image quality, a method for improving viewing angles has been studied, developed, and then become commercially practical. As a result, the mainstream of present high-performance liquid crystal displays are divided into three types, that is, TN-TFT type active matrix liquid crystal display apparatuses using compensated films, TFT type active matrix liquid crystal display apparatuses of an in-plane switching (IPS) mode, and TFT type active matrix liquid crystal display apparatuses of a multi-domain vertical aligned (MVA) mode.
In these active matrix liquid crystal display apparatuses, an image signal having a frequency of 30 Hz is generally used and refreshed by the frequency of 60 Hz for positive-negative writing. A time taken for one field is about 16.7 milliseconds (ms); that is, the total time taken for the positive and negative fields is called one frame and is about 33.3 ms. In contrast, with the response speed of present liquid crystal display apparatuses, even their fastest response speed is only represented as such a frame time even with consideration given to responses during their intermediate gradation display. Because of this, to display video signals of moving images, high-speed computer graphics (CG), and high-speed game images, it is necessary to secure a faster response speed than that represented by the present frame time.
In addition, dominant pixel sizes are on the order of 100 ppi (pixels per inch) at present and higher definition is achieved by the two methods described below. One method is a method for decreasing pixel sizes through enhanced processing accuracy and the other is a method for fabricating a field-sequential (time-sharing) color liquid crystal display apparatus in which a backlight serving as the illuminating light of the liquid crystal display apparatus is switched in time sequence among red, green, and blue and at the same time, red, green, and blue images are displayed. In the latter method, since there is no need to spatially dispose a color filter, it is possible to achieve definition three times as high as the conventional ones. In the field-sequential liquid crystal display apparatus, there is a need to display one color in a time corresponding to one-third of one field and hence, a time usable to display the color is about 5 ms. Therefore, the liquid crystal itself is required to respond in less than 5 ms.
From the need for such a high-speed liquid crystal, various techniques have been studied and several high-speed display mode technologies have been developed. These high-speed liquid crystal technologies are broadly divided into two trends. One of these is a technique for enhancing the response speed of the foregoing dominant nematic liquid crystals and the other is a technique for employing spontaneous polarization-type smectic liquid crystals capable of responding at high speed and so on. The first trend, that is, the enhancement of the response speed of the nematic liquid crystals is mainly effected by the following methods: (1) cell gaps are reduced to increase an electric field strength through the application of the same voltage; (2) a high voltage is applied to increase the electric field strength and to promote a change in the state of the liquid crystal (an overdrive system); (3) the viscosity of the liquid crystal is lowered; (4) a mode, which is considered to have high-speed responsivity in principle, is used, and so on.
Even in such high-speed nematic liquid crystals, the following problems arise. In the high-speed nematic liquid crystal, since liquid crystal responses are almost completed within its frame, a change in the capacity of the liquid crystal layer remarkably increases due to the anisotropy of its dielectric constant. Due to the change in the capacity, a holding voltage to be held by writing to the liquid crystal layer changes. Such a change in the holding voltage, that is, a change in an effective applied voltage makes the contrast lowered due to insufficient writing. And when the same signal is written continuously, brightness continues to change until the change in the holding voltage ceases and hence, several frames are required to obtain stable brightness.
To prevent such responses requiring several frames, it is necessary to establish a one-to-one correspondence between an applied signal voltage and an obtained transmittance. In active matrix driving, a transmittance after a liquid crystal response is determined by the amount of charge accumulated in a liquid crystal capacitor after the liquid crystal response instead of a signal voltage is applied. This is because active driving is constant-charge driving in which a liquid crystal is made to respond by a held charge. The amount of a charge supplied from the active element is determined by a charge accumulated before a predetermined signal is written and a write charge newly written when its minute amount of leakage and so on are ignored. In addition, a charge accumulated after the liquid crystal responds also changes through the physical property constant of the liquid crystal and pixel design values such as an electrical parameter and a storage capacitor. Because of this, to establish the correspondence between the signal voltage and the transmittance, the followings are necessary: (1) correspondence between the signal voltage and the write charge, (2) the accumulated charge before the writing, and (3) acquiring information for calculating the accumulated charge after the response and calculating actually. As a result, it is necessary to provide a frame memory used for storing the item (2) across an entire screen and a calculating unit used for calculating the items (1) and (3).
On the other hand, as a method of establishing the one-to-one correspondence without the use of the foregoing frame memory and calculating unit, a reset pulse method is often used in which a reset voltage is applied to align liquid crystals into a predetermined state before data is newly written. As an example, a technique described in IDRC 1997 pp. L-66 to L-69 (hereinafter referred to as “second publication”) will be explained. In the second publication, an OCB (optically compensated birefringence or optically compensated bend) mode is used in which the alignment of the nematic liquid crystal is a pie-type alignment and a compensated film is added. A response speed in the liquid crystal mode is on the order of 2 to 5 milliseconds and therefore significantly faster than that in conventional TN mode. As a result, a response should essentially complete within one frame, while as described above, since a significant decrease in a holding voltage occurs due to a change in a dielectric constant resulting from the response of the liquid crystal, several frames are required until a stable transmittance can be obtained. In view of this, a method for always writing a black image after the writing of a white image within one frame is shown in FIG. 5 of the second publication. This figure will be quoted as FIG. 4. In FIG. 4, a horizontal axis represents time and a vertical axis represents brightness. A dotted line represents a change in brightness during normal driving and a stable brightness is reached at the third frame. According to the reset pulse method, since a predetermined state is always secured at the time of the writing of new data, a one-to-one correspondence between a certain written signal voltage and a certain transmittance can be observed. Through the one-to-one correspondence, driving signals are generated very easily and a unit such as a frame memory, which stores previously written information, becomes unnecessary.
The configuration of a pixel of an active matrix liquid crystal display apparatus will be summarized below. FIG. 1 is a circuit diagram of one of the pixels of a conventional active matrix liquid crystal display apparatus. As shown in FIG. 1, the pixel of the active matrix liquid crystal display apparatus comprises a MOS transistor (Qn) (hereinafter referred to as “transistor Qn”) 904, a storage capacitor 906, and a liquid crystal 908. The MOS transistor (Qn) 904 has a structure in which a gate electrode is connected to a scanning line 901 (or a scanning signal electrode), either a source electrode or a drain electrode is connected to a signal line 902 (or a video signal electrode), and the remainder of these is connected to a pixel electrode 903. The storage capacitor 906 is formed between the pixel electrode 903 and a storage capacitance electrode 905. The liquid crystal 908 is sandwiched between the pixel electrode 903 and an opposing electrode (or a common electrode) Vcom 907.
In notebook personal computers (notebook PCs) which form a large application market for liquid crystal display apparatuses at present, an amorphous silicon thin film transistor (a-Si TFT) or a polycrystalline silicon thin film transistor (p-Si TFT) is used as the transistor (Qn) 904 and a TN liquid crystal is used as the liquid crystal material in general. FIG. 2 is an equivalent circuit diagram of a TN liquid crystal. As shown in FIG. 2, the equivalent circuit of the TN liquid crystal can be represented as a circuit in which the capacitive component C3 (its electrostatic capacitance Cpix) of the liquid crystal is connected in parallel to a resistor R1 (its resistance Rr) and a capacitance C1 (its electrostatic capacitance Cr). In this equivalent circuit, the resistance Rr and the capacitance Cr are components which determine the response time constant of the liquid crystal.
A timing chart of a scanning line voltage Vg, a signal line voltage (or video signal voltage) Vd, the voltage of the pixel electrode 903 (hereinafter referred to as “pixel voltage”) Vpix, which are obtained by driving such a TN liquid crystal by using the pixel circuit shown in FIG. 1, is shown in FIG. 3. As shown in FIG. 3, by raising the scanning line voltage Vg to a high level VgH during a horizontal scanning period, the n-type MOS transistor (Qn) 904 is turned on and then the signal line voltage Vd, which is inputted into the signal line 902, is transferred to the pixel electrode 903 via the transistor (Qn) 904. The TN liquid crystal generally operates in a mode in which light passes through during the period without applying a voltage, i.e., a so-called normally white mode.
In this case, as the signal line voltage Vd, a voltage, by which the transmittance of light which passes through the TN liquid crystal is enhanced, is applied over several fields. When the horizontal scanning period has completed and the scanning line voltage Vg has been brought to a low level, the transistor (Qn) 904 is turned off, thereby the signal line voltage transferred to the pixel electrode 903 is held by the storage capacitor 906 and the capacitance Cpix of the liquid crystal. The pixel voltage Vpix shows voltage shifts called feed-through voltages via the gate-to-source capacity of the transistor (Qn) 904 at a time when the transistor (Qn) 904 is turned off. In FIG. 3, the voltage shifts are represented as Vf1, Vf2, Vf3. The amounts of the voltage shifts Vf1 to Vf3 can be decreased by designing the storage capacitor 906 so as to stand at a large value.
The pixel voltage Vpix is held during the next field period until the scanning line voltage Vg is brought to the high level again and the transistor (Qn) 904 is selected. The switching of the TN liquid crystal is created according to the held pixel voltage Vpix; that is, as shown as a light transmittance T1, the transmitted light of the liquid crystal transitions from a dark state to a bright state. At this point in time, as shown in FIG. 3, the pixel voltages Vpix vary at the individual fields by ΔV1, ΔV2, ΔV3 respectively during the holding period. This results from a fact that the capacity of the liquid crystal varies according to the response of the liquid crystal. To minimize the variation, the storage capacitor 906 is generally designed in such a way that it stands at a large value which is at least 2 to 3 times that of the pixel capacity Cpix. As explained above, the TN liquid crystal can be driven by using the pixel circuit shown in FIG. 1.
As a technique having an effect achieved by using a method developed by combining the overdrive system and the reset system, there is a technique of modulating a common voltage (common electrode voltage (or opposing electrode voltage)) shown in Japanese Translation of International Application (Kohyo) No. 2001-506376 (hereinafter referred to as “first publication”). FIG. 2C of the first publication is quoted as FIG. 5. In this technique, the common voltage, which is a voltage at a common electrode disposed so as to be opposite a pixel electrode, is modulated in general. In FIG. 5, the upper graph shows a variation in common voltage (VCG) with respect to time and the lower graph shows a variation in light transmittance (I) caused by the response of the liquid crystal with respect to time. That is, a voltage waveform 151 represents the waveform of a voltage applied to the common electrode, a light intensity waveform 152 represents a light intensity waveform corresponding to the waveform 151 with respect to time, and line segments 153 to 156 represents pixel light intensity curves. In techniques used prior to the use of this technique, driving during which a common voltage is held at a constant value is conducted or common reverse driving is conducted in which voltage varies so as to take on two values in a constant cycle represented as one frame cycle which comprises respective periods of t0 to t2 and t2 to t4 shown in FIG. 5. In the first publication, one frame cycle is divided into halves during the respective periods of t1 to t2 and t3 to t4, a voltage whose amplitude is the same as that used in conventional common reverse driving is applied. On the other hand, during periods of t0 to t1 and t2 to t3 of one frame cycle, a voltage higher than the amplitude of the common reverse such as a voltage higher than the amplitude of the common reverse by a voltage generated during a black image is applied. In this technique, there is an effect that a voltage differential between the pixel electrode and the common electrode increases during the period of t0 to t1 over which a high voltage is applied to the common electrode, thereby the entire display region can be rapidly changed to a black image. That is, driving corresponding to the reset driving is performed. Moreover, even when image data is written into the pixel electrode during the period of t0 to t1, the writing is not observed on the display since a potential difference between the pixel electrode and the common electrode is sufficiently large (for example, a voltage placed for the black image or larger). After image data is written into the entire display region, the voltage at the common electrode is returned to the amplitude of the common reverse with a timing of t1. As a result, the liquid crystal layer initiates its responses in such a way that its transmittance varies according to respective gradation levels based on the voltage stored in the pixel electrode. That is, at the time of the initiation of the response, the state in which the voltage differential is large always changes to a state in which voltage differentials are coincide with voltages at the respective gradations. Therefore, a kind of overdrive occurs during the period of t0 to t1.
Here, it should be noted that the response time of liquid crystal is generally given by the following two expressions (see the Dictionary of Liquid Crystal, p. 24, published by Baifukan Ltd., edited by the Japan Society for the Promotion of Science, the 142nd Committee on Organic Materials for Information Science, the Group on Liquid Crystal, and hereinafter referred to as “third publication”): that is, in a rise response (on-time response) in which a voltage which is higher than a threshold voltage is applied to effect an on state, the following expression 1 is established:
                              τ          rise                =                                            d              2                        ·                          η              ~                                            Δ            ⁢                                                  ⁢                          ɛ              ·                              (                                                      V                    2                                    -                                      V                    c                    2                                                  )                                                                        (                  Expression          ⁢                                          ⁢          1                )            
On the other hand, in a fall response (off-time response) in which the applied voltage which is higher than the threshold voltage is quickly brought down to zero V, the following expression 2 is established:
                              τ          decay                =                                            d              2                        ·                          η              ~                                                          π              2                        ·                          K              ~                                                          (                  Expression          ⁢                                          ⁢          2                )            where d is the thickness of a liquid crystal layer, η is a rotation viscosity, Δ∈ is dielectric anisotropy, V is an applied voltage corresponding to each gradation level, Vc is a threshold voltage, and K is a constant based on Frank elastic constant. In TN mode, K is given by the following expression 3:
                              K          ~                =                              K            11                    +                                    1              4                        ⁢                          (                                                K                  33                                -                                  2                  ·                                      K                    22                                                              )                                                          (                  Expression          ⁢                                          ⁢          3                )            where K11 is the elastic constant of a spread, K22 is the elastic constant of a twist, and K33 is the elastic constant of a bend. As is apparent from the expression 1, in the rise response (on-time response), the response time of liquid crystal depends on the reciprocal of the square of the value of an applied voltage. That is, the response time of the liquid crystal depends on the reciprocal of the square of the value of a voltage which varies according to each gradation level. Because of this, the response time significantly varies according to the gradation levels; when there is a ten-times voltage differential, a hundred-times difference in the response time occurs. On the other hand, even in the fall response (off-time response), there is a difference in the response time according to the gradation levels; however, the difference falls within an about double range.
According to the third publication, the response speed of the liquid crystal is increased by the overdrive effect that a very high voltage is applied at the time of the rise response (on-time response). Moreover, since every responses used for actual image displays are fall responses (off-time responses), a dependence on the gradation levels is remarkably low. As a result, about the same response time can be achieved over all gradations.
However, the foregoing liquid crystal display apparatuses, that is, the display apparatus using the overdrive, the display apparatus using the reset drive, the display apparatuses disclosed in the documents such as the first publication, and so on have several problems.
A first problem is as follows: in the overdrive system, the rise response (on-time response) speed of the liquid crystal can be increased, while the response speed is on the order of ten and several milliseconds to several tens of milliseconds at most due to limited materials for the liquid crystal. Moreover, as described below, the fall response (off-time response) speed cannot be increased so much.
Such a problem can be solved by the following means. To increase the response speed of the liquid crystal itself, as is apparent from the expressions 1 and 2, it is preferable to take effective measures such as the following:    (1) decreasing the thickness d of the liquid crystal layer;    (2) lowering the viscosity η;    (3) enhancing the dielectric anisotropy Δ∈ (only in the rise (on-time) response);    (4) increasing the applied voltage (only in the rise (on-time) response); and    (5) decreasing the elastic constants K11 and K33 and increasing the elastic constant K22 (only in fall (off-time) response). However, with the item (1), to sufficiently achieve an optical effect, the thickness of the liquid crystal layer can be decreased only within the range of its constant relationship with a refractive index anisotropy Δn. Moreover, with the items (2), (3), and (5), since all viscosity, the dielectric anisotropy, and the elastic constants are physical property values, these are highly dependent on the materials of the liquid crystal and hence, it is difficult to set these at values which exceed certain conditions. And further, it is very difficult to significantly vary any one of the physical property values and hence, it is difficult to achieve an effect on the high-speed response expected from the expressions. For example, although K11, K22, and K33 are independent elastic constants, a relationship K11:K22:K33=10:5:14 is substantially established from the measurement results on actual materials, so that these cannot be always treated as independent constants. That is, from the relationship and the expression 3, for example, a relationship K=11·K22/5 is established and therefore, only K22 is an independent constant. Because of this, these can be adjusted a little, but it is difficult to achieve improvement above scores of percent. Still further, with the item (4), the method for increasing the value of the applied voltage also has a considerable limitation in terms of power consumption and the increased production cost of the high-voltage driving circuit. In addition, when the display apparatus is driven by providing an active element such as a thin film transistor, the response speed is limited by the withstand pressure of the element. As described above, there is a considerable limit to the fast response speed achieved by using those conventional means including the overdrive in principle.
A second problem is that in the overdrive system, the rise response (on-time response) can be sped up but the fall response (off-time response) can be hardly sped up. As is apparent from the expressions 1 and 2, this is because the rise response (on-time response) depends on the potential difference to effect the variation in the response time but the fall response (off-time response) does not depend on the potential difference. That is, the rise response (on-time response) can be sped up by increasing the potential difference, but the fall response (off-time response) cannot. As a result, in the conventional overdrive system, the fall response (off-time response) not sped up dominantly determines the response speed of the entire system.
A third problem is that in the conventional overdrive system the voltage required for the overdrive is high. The video signals of the display apparatus are high-frequency signals and hence, in the overdrive system in which the voltages of the video signals are increased, power consumption, which is determined by the voltage and the frequency, has been increased significantly. Moreover, since there is a need to produce the high-frequency high-voltage signals, it is difficult to use the same driving IC and signal conditioning system as those of conventional display apparatuses, so that a need to use ICs fabricated by using a special process or expensive ICs has often arisen.
A fourth problem is that in the reset system, a method of applying reset signals via a pixel switch has the disadvantages that the structure of a driving system becomes complex and power consumption is increased. That is, scanning for the writing of the video signals requires the driving of scanning lines which is different in scanning period and scanning method. When the pixel switch is reset, a method for resetting all scanning lines together is often used instead of sequential scanning and hence, it becomes necessary to provide a structure where signals are sent together into the scanning system. Moreover, since the scanning lines are driven at the time of not only the writing of the video signals but also the writing of the reset signals, the frequencies of signals for the scanning lines having the highest voltage amplitude in the display apparatus are increased, thereby power consumption is increased. As a result, it is desirable that the reset not be conducted via the pixel switch.
A fifth problem is that in the reset system, the state of the display considerably changes due to the reset of an excessive or short degree. This problem also holds true for the method described in the first publication which is created by combining the overdrive system and the reset system.
First, the reset is excessive, the initiation of the optical response of the liquid crystal after the reset becomes slow and abnormal optical responses are observed before the initiation of normal optical responses. This is because at the time of a transition from a predetermined alignment state realized by the reset to the normal response, a direction in which the liquid crystal operates during the response is not clear and hence, nonuniform and unstable responses are shown. An example of the abnormal optical responses is shown in FIG. 21. As shown in FIG. 21, when the reset is excessive, delays in the optical responses and abnormal displays (such as the transient rise in transmittance) develop.
On the other hand, in the reset system, the shortage of the reset effects a situation where when the same data is written several times, the same transmittance cannot be sometimes obtained. When the reset is insufficient, a predetermined alignment state is not completely realized during the reset, so that the response following the reset shows transmittance corresponding to the history of a previous frame. As a consequence, a one-to-one correspondence is not established between the applied voltage and the transmittance. Because of this, a desired gradation cannot be attained or even when the same gradation is displayed, brightness varies greatly. The variation in the brightness may result in, for example, a difference between brightness caused by the application of a positive signal voltage and brightness caused by the application of a negative signal voltage, that is, flicker.
A sixth problem is that it is difficult to attain stable display over a wide temperature range. This is because the viscosity η of the liquid crystal is highly dependent on temperature and hence, the response speed of the liquid crystal is also highly dependent on temperature. Particularly, in the reset system and the method described in the first publication, when a temperature changes, the foregoing excessive or insufficient reset develops clearly. As a result, the response speed is decreased at low temperatures, which result in, for example, a considerable reduction in brightness. On the other hand, at high temperatures, for example, the response speed at intermediate gradation display is increased and the brightness is enhanced all over the display, so that the display approaches a white image. Because of this, a phenomenon in which the entire display becomes whitish and so on takes place. Furthermore, since the shortage of the reset occurs at low temperatures, the problem that the correspondence between the applied voltage and the transmittance is not established arises, thereby a desired gradation cannot be obtained or flicker is caused.